Gourang made an electric circuit with three bulbs `B_(1), B_(2)` and `B_(3)` a cell and two switches `S_(1)` and `S_(2)` as shown in figure. He notes his observation when

(a) `S_(1)` is closed and `S_(2)` is open,
(b) `S_(2)` is closed and `S_(1)` is open.
He also found that a bulb whose failure made the circuit 'open' irrespective of the status of `S_(1)` and `S_(2)` . What are his observation.

Explanation

(a) If `S_(1)` is closed and `S_(2)` is open, only `B_(1)` and `B_(2)` glow.
(b) If `S_(2)` is closed and `S_(1)` is open, only `B_(1)` and `B_(3)` glow.
If `B_(1)` fails the circuit becomes open, even if both `S_(1)` and `S_(2)` are closed.